Define/use an additional secret key, Refactor token replacement for signed urls

[qqmyers]

What this PR does / why we need it: The two commits in this PR do two different things:

1. The initial use case for signed URLs used a secret shared between Dataverse and the remote store to create/validate signed urls. In the initial dev for signed URLs for external tools, we swapped to using the apiKey of the intended user which has value in avoiding a single key across a whole Dataverse instance. However, the apiKey is relatively short and is known to the user (making it possible for the user or someone who gains that apiKey) to sign URLs associated with their account. This PR adds a dataverse.api-signing-secret jvm option that is prepended with the user's apiKey when signing, which then a) provides a longer, more secure signing/validation key, and b) maintains a one-key-per-user model (that can be changed by the user by invalidating/recreating their apiKey) while avoiding using something the user can access/obtain.

The external tools signing will also have to use the same aggregate key (global secret + user api key) when creating signedUrls when launching external tools. (Perhaps a util method to get the key that gets used in all these places?).

2. The original code to replace ReservedWord tokens was limited to use as part of the external tools framework and only handled creating query parameters. For openDP/signedURLs, we also want to replace tokens in URLs (i.e. path parameters). For HDC Objective 1/Globus integration, it is also useful to be able to replace tokens rather than having to write custom code to do so. The second commit here refactors the externalTools framework to pull the reserved words and the basic token replacement into a new URLTokenUtil class and adds, in that class, a new method to substitute tokens in a string/URL to handle the case where path parameters need to be entered.

[coveralls]

Coverage increased (+0.03%) to 18.901% when pulling 208ab95 on GlobalDataverseCommunityConsortium:7715-signed-urls-for-external-tools into 01973ff on IQSS:7715-signed-urls-for-external-tools.