Add support for /transactions endpoint

[bradlo]

get /transactions
post /transactions
get /transactions/{txid}
delete /transaactions/{txid}
get /transactions/{txid}/metadata
get /transactions/{txid}/results
post /transactions/{txid}/cancel

[billscheidel-rai]

Note: This issue has been migrated to https://relationalai.atlassian.net/browse/RAI-4445.

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