Improve user agent parsing (Sentry not always reporting UA data correctly)

[pjenvey]

We use the generic woothee user agent parser, however our Sync clients tend to use custom UA strings that don't always parse down to useful tags (e.g. #787).

We should implement our own user agent parsing instead. Either woothee should be abandoned entirely or only used as a fallback.

[rfk]

Aye, unfortunately most standard UA-parsing libraries don't handle the sync UA strings well. Summary of known current client behaviour:

• Firefox Desktop tries to send a normal-looking UA string, with some extra information about sync appended to it, see here.
• The user-agent logic for Firefox for iOS lives here; it tries to send a fairly usual-looking UA string but prepends a unique token like "Firefox-iOS-Token" or "Firefox-iOS-Sync" depending on what service it's trying to access.
  • However! For reasons, requests from Firefox for iOS that originate in the shared rust components will use a hard-coded user-agent string of Firefox-iOS-FxA/24.
• Android clients all use the HTTP client library from android-components, which sets the default user-agent string to MozacFetch/${BuildConfig.LIBRARY_VERSION}; I don't believe any clients currently alter this from its default.

For unrelated reasons, I'm interested in trying to get all our clients to include additional information in their UA string, such as the release channel identifier. I think that might argue in favor of using some custom parsing to augment the off-the-shelf library.

[pjenvey]

More observation of the UA strings in the duplicate #1027, such as this comment