feat: Proposal for re-designing the AWS KMS keyring (#148)

* feat/docs: Proposal for re-designing the AWS KMS keyring

* nit: Formatting

* feat!: Remove AWS KMS symmetric all region discovery keyring from proposal to avoid region staleness issues

* docs: Update features, spec, affected implementations links to they're relative instead of absolute

* fix: Updated language used

* fix: Update language, clarify change doc per PR feedback

* docs: Add term for 'derived keyrings'

* nit: grammar fix

* fix: Missed adding Spec issue #49 to list of affected features

* docs: Further clarification around optional params and multi-keyrings

* nit: Add 2 additional issues handled by PR

* docs: Add lack of any region discovery keyring to drawbacks

* docs: Further clarify user agent string

* fix: Add SHA to kms-keyring link in features

Author: acioc