Add expiration date and time for bindings

[fmui]

What is the problem this PR solves?

The PR adds an optional expiration date and time to a binding. It should indicate applications and platforms that this binding should not be used anymore after this date. It may also be used to initiate automatic rotation of bindings and credentials.

The new expires_at field is enclosed in a new metadata object, which may be used for other binding management data in the future.

Checklist:

• [x] The swagger.yaml doc has been updated with any required changes
• [x] The openapi.yaml doc has been updated with any required changes

[linux-foundation-easycla[bot]]

The committers are authorized under a signed CLA.

• :white_check_mark: Florian Müller (20245e93394d57906791c6b1219d5fed1b4933ad, ffb2ca5c407a0b0eb50950444c5d06bdc1b35385, bcb3b46f1e00c3fa4a5771a06403f53dd21340dd, f47b8dd54e827f020d35d77717a1e272c63cee66)

[mattmcneeney]

@tinygrasshopper to review