[PORT] [DCR] Normalize all expression and add '=' to force expression.

[tomlm]

Port this change from botbuilder-dotnet/master branch:
microsoft/botbuilder-dotnet#3253

• Added StringExpression,ValueExpression, etc. for expression or value access
• added a ton of unit tests for the expression stuff
• For ambigious expressions (StringExpression and ValueExpression) it will treat as string interpolation unless it starts with =. "Hello @{world}" ==> "hello world", "=user.foo" => Expression("user.foo")
• Added assignment operators so you can assign string expressions to expression properties.
• cleaned up dialogs files
• Exposed Dialogset to DialogManager so you can register external external dialogs
• Added DialogId to BeginDialog to allow you to invoke external dialogs

It basically boils down to how to deal with strings as input:

• StringExpression always assumes string interpolation unless prefixed with =, producing a string
• ValueExpression always assumes string interpolation unless prefixed with =, producing an object which MAY be a string
• ExpressionProperty<T> always assumes strings represent something to be interpreted as the type of T, so always producing object of T.

StringExpression

Interpretation of string is as a string interpolation result unless =.

| Input                         | Result        |
|-------------------------------|---------------|
| "13"                          | "13"          |
| "=13"                         | "13"          |
| "user.age"                    | "user.age"    |
| "=user.age"                   | "13"          |
| "Hello world"                 | "Hello World" |
| "='Hello world'"              | "Hello World" |
| "Hello @{user.name}"          | "Hello Joe"   |
| "=concat('Hello ', user.name) | "Hello Joe"   |

BoolExpression

Interpretation of string is as a bool expression

| Input           | Result        |
|-----------------|---------------|
| true            | true          |
| "true"          | true          |
| "=true"         | true          |
| "user.age > 3"  | true          |
| "=user.age > 3" | true          |
| "bad"           | default(bool) |
| "bad @{bad}"    | default(bool) |

IntExpression

Interpretation of string is as a int expression

| Input                | Result       |
|----------------------|--------------|
| 13                   | 13           |
| "13"                 | 13           |
| "=13"                | 13           |
| "x.y.z"              | 13           |
| "=x.y.z"             | 13           |
| "Hello world"        | default(int) |
| "Hello @{user.name}" | default(int) |

FloatExpression

Interpretation of string is as a float expression

| Input        | Result         |
|--------------|----------------|
| 3.14F        | 3.14           |
| "3.14"       | 3.14           |
| "=3.14"      | 3.14           |
| "x.y.z"      | 3.14           |
| "=x.y.z"     | 3.14           |
| "bad"        | default(float) |
| "bad @{bad}" | default(float) |

ExpressionProperty

Interpretation of string is as a T expression

| Input        | Result     |
|--------------|------------|
| T            | T          |
| "x.y.z"      | T          |
| "=x.y.z"     | T          |
| "bad"        | default(T) |
| "bad @{bad}" | default(T) |

ValueExpression (T == object)

Interpretation of string is as a string interpolation result unless =.

| Input                         | Result      |
|-------------------------------|-------------|
| object                        | object      |
| "13"                          | "13"        |
| "=13"                         | 13          |
| "x.y.z"                       | "x.y.z"     |
| "=x.y.z"                      | object      |
| "='x.y.z'"                    | "x.y.z"     |
| "Hello @{user.name}"          | "Hello Joe" |
| "=concat('Hello ', user.name) | "Hello Joe" |

Also Fixes Bugs
#3166 Luis settings not parsed correctly
#3236 Use dialogId as an expression
#3219 HttpRequest body not parsed correctly

[luhan2017]

resolved