THRIFT-5471 Introduce delimiter and app_prefix options to erl generator

[kpy3]

Option app_prefix help to resolve module name clashes for cases where,
some structures and/or services uses the same names for different applications.

Option delimiter enable to redefine hardcoded delimiter between namespace and
structure name (avoiding duplications if any).

• Did you create an Apache Jira ticket? (not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[Jens-G]

Please file a ticket next time for such changes, otherwise it will not appear in the release CHANGES list.