Set user agent at different layers for modular

[timovv]

~~Update user agent to change depending on which layer is being used (TODO: update these to add -modular)~~

• ~~If the classic client layer is used, azsdk-js-<packageName>-classic/<version>~~
• ~~If the modular API layer is used, azsdk-js-<packageName>-api/<version>~~
• ~~REST level remains unchanged azsdk-js-<packagename>-rest/<version>~~

~~The azsdk-js- prefix is omitted for unflavored packages.~~

~~Problem: RLC currently concatenates the passed in userAgentPrefix so we may get long user agents like azsdk-js-<packageName>-api/<version> azsdk-js-<packagename>-rest/<version>~~

Implement user agent string as defined in this Loop:

For a package @azure/my-library version 1.0.0:

• Requests from the REST layer will have user agent prefix azsdk-js-my-library/1.0.0
• Requests from the API layer will have user agent prefix azsdk-js-api azsdk-js-my-library/1.0.0
• Requests from the client layer will have user agent prefix azsdk-js-client azsdk-js-api azsdk-js-my-library/1.0.0