Remove quotes around _expected and _since values

[alexcottner]

Right now, remote-settings clients can (optionally) send quotes around _expected and _since.

We should:

1. Remove quotes from the client calls in firefox (js client)
2. Remove quotes from the rust client calls in firefox (rust client)
3. Allow the v1 endpoints to be forwards compatible (allow for optional quotes)
4. Make v2 endpoints strict (don't allow quotes)

[leplatrem]

Some places that would be impacted too:

https://searchfox.org/firefox-main/rev/911b3eec6c5e58a9a49e23aa105e49aa76e00f9c/third_party/rust/remote_settings/src/client.rs#591-602

https://remote-settings.readthedocs.io/en/latest/client-specifications.html#cache-busting

[alexcottner]

Service-side work is all done.
Phab patch open: https://phabricator.services.mozilla.com/D277258
Dev tools PR is open: mozilla-extensions/remote-settings-devtools#329