Optimize MSGraph OpenApi specs caching based on e-tag

[bartizan]

Based on Waldek's suggestion, a MS Graph OpenAPI specification should be downloaded from the network only if a newer version with a different E-tag is available.

Key concept

• Once a day, DevProxy checks for a newer OpenAPI spec version based on the LastWriteTime file attribute and E-tag;
• The MS Graph OpenApi spec is downloaded and stored on the disk along with the E-tag value (in a *.etag.txt file);
• Try to clean up stored resources that cannot be parsed or accessed.

[waldekmastykarz]

Sounds like a plan! Would you like to give it a try @bartizan?

[bartizan]

Sounds like a plan! Would you like to give it a try @bartizan?

I do.
I have got on it.