* separate tenantID from m365 creds
Now that account.Account is in place, tenant id needs
to get removed from the credential set (it isn't actually
a secret) and placed in the account configuration instead.
* move config unions to common code
The configuration union handlers in Storage and Account
overlapped significantly in behavior. Moving those helpers into
a common code folder was requested. Although the behavior
was similar across the files, the types were not, requiring
the addition of generics to solve the need.
* add Account pattern to mimic storage
Introduces /pkg/account, which produces a generic Account
struct that can be configured from specific provider
details (such as m365 credentials), and serialized back to
those details as needed.
Next steps include replacing repository.Account with the
new account. After that, tenantID should get removed from
the m365 credentials handling and placed into the Account
configuration instead.
