Allow passing FixedSpan to TLVWriter::Put.

[bzbarsky-apple]

Two main changes here:

1. Narrow down the existing Put template for enums to only get matched
   for enum types, so it does not try to get compiled for a FixedSpan.

2. Introduce more inter-conversions between Span and FixedSpan, where
   possible, via implicit constructors.

Problem

Can't Put a FixedSpan, so people hand-write the data() and size() bits.

Change overview

See above.

Testing

Passes unit tests, and I can now pass a FixedSpan to TLVWriter::Put (in a different PR).

[github-actions]

Size increase report for "esp32-example-build" from 47823f1

File	Section	File	VM
chip-temperature-measurement-app.elf	.flash.text	-60	-60
chip-lock-app.elf	.flash.text	64	64
chip-shell.elf	.flash.text	4	4

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-persistent-storage.elf and ./pull_artifact/chip-persistent-storage.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-temperature-measurement-app.elf and ./pull_artifact/chip-temperature-measurement-app.elf:

sections,vmsize,filesize
[Unmapped],0,60
.debug_line,0,2
.debug_str,0,2
.flash.text,-60,-60

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_str,0,7
.debug_line,0,1
.debug_loc,0,-16

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-lock-app.elf and ./pull_artifact/chip-lock-app.elf:

sections,vmsize,filesize
.flash.text,64,64
.debug_line,0,2
.debug_str,0,2
.debug_loc,0,-48
[Unmapped],0,-64

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.flash.text,4,4
.debug_str,0,3
.debug_line,0,1
[Unmapped],0,-4

[bzbarsky-apple]

@jmartinez-silabs @LuDuda @jepenven-silabs @msandstedt @saurabhst please take a look?