Implement local caching for WMTS requests

[dnowacki-usgs]

This is an in-progress PR to implement local caching of WMTS image tiles. It is based heavily on the caching approach and code implemented in #1533. No tests added yet; this is really a working proof of concept.

Rationale

The rationale is described in #2314.

Briefly, this PR implements local caching of image tiles so repeated WMTSRasterSource calls do not incur additional network traffic.

Implications

• Reduced network usage for repeated calls.
• Creation of files on disk in a temporary directory.
• Ability to work offline and use WMTSRasterSource

[greglucas]

Seems reasonable. There is a lot of duplicated code here between this and the other cache PR, so I am curious if there would be a good way to consolidate some of it into helper functions or class to be shared somehow. That can be a follow-up PR as well though if you don't want to expand the scope here.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[dnowacki-usgs]

Thanks for your feedback. Working on implementing these suggestions!