feat: use default credentials for keyspaces

[CallumNZ]

Reference: https://github.com/GeoNet/tickets/issues/12494

Change to allow aws roles for authentication to Keyspaces.

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[wilsonjord]

question: seems like it's replacing one style of authenication (with KEYSPACE_* variables set), with one based on AWS roles...assume risk of loads of code being broken is small?

[CallumNZ]

tilde-ws and tilde-meta are the only ones that use it.
I'll be updating their respective terraform/code after grabbing the new version of kit.

[wilsonjord]

Cool thanks. Well, this PR looks good. Incidentally, I'll also be working on kit updates which may be "breaking". Perhaps a future discussion on versioning might be handy.