Access Token Proof-of-Possession for msal-browser

[pkanher617]

This PR merges all of the previous PRs for access token proof-of-possession to the dev branch to prepare for release tomorrow.

The following PRs are included:

• AT Proof-Of-Possession #1: Adding req_cnf to the token request #2151 Adding req_cnf to the request
• AT Proof-Of-Possession #2: Handle proof-of-possession tokens correctly in responses #2153 Handle proof-of-possession tokens correctly
• AT Proof-Of-Possession #3: Browser Implementation of token signing and key storage #2154 Browser implementation of key signing and storage
• AT Proof-Of-Possession #4: Renewal of Proof-of-possession tokens #2209 Renewal of proof-of-possession tokens

[coveralls]

Coverage decreased (-0.9%) to 82.799% when pulling 65db2f0 on pop into 97a802a on dev.

[jmckennon]

I am assuming that there aren't any other changes besides the ones included from the linked PRs which were already approved, so LGTM!

[pkanher617]

I am assuming that there aren't any other changes besides the ones included from the linked PRs which were already approved, so LGTM!

Just bug fixes and sample updates.