feat(internal/serviceconfig): centralize API allowlists (#3523)

A centralized allowlist is introduced within
internal/serviceconfig/allowlist.go.

The allowlist defines the core set of API paths for which client
libraries can be created across all languages.

A legacy allowlist is also created for language-specific exceptions,
which is initially populated with Python APIs that are not part of the
global allowlist.

The data from this list will be used by `librarian add` and `librarian
generate` in a follow up PR.

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Signed-off-by: Julie Qiu <julie@golang.org>
Co-authored-by: Carlos O'Ryan <coryan@google.com>

Author: julieqiu