Kate/server connection fixes

[KatKatKateryna]

Small improvements:

• date-to-string and date-from-string functionality is centralized for Diff and Save - no variations in different places. Also 'self._diff_original_and_current_data' will return both boolean and the final data dictionary, so we don't need to call 'asdict_enum_safe' inside of it, and then later again.
• accept differently formatted datetime strings on Open - when we receive server data it's being received with '+00:00' at the end instead of 'Z'
• ensure sending utf-8 encoded data to server admin

Remaining issues:

1. Server admin incorrectly decodes the characters, while payload is encoded and sent correctly from the plugin:

2. Datetime format is being changed on admin side, although we explicitly send correctly formatted strings:

3. Unit tests are missing - can be added through github action forking and deploying pygeoapi server from the official repo.

[doublebyte1]

Small improvements Keeping as draft until other datetime issues are resolved

It seems with your changes the dates are still working fine (thanks for removing the redundant code). Are there any specific use cases where you see the dates failing?

[KatKatKateryna]

Some inconsistent behaviors, checking now 🙏

[KatKatKateryna]

@doublebyte1 finished the edits and adjusted PR description 🙏
P.S. opened related PR in pygeoapi repo: geopython/pygeoapi#2191