[RFC] Default to `GET` for item search?

[gadomski]

Since POST is recommended but not required, it can be surprising to users when pystac-client defaults to POST (e.g. #641). On the other hand, any sort of complicated intersects will probably blow up a GET request. I'm not quite sure what the most user-friendly logic is here, anyone have thoughts?

[gadomski]

From @philvarner in Slack:

landing page link -- if it has a link with rel=search without a method field, it assumed to be GET
a link with method = POST defines that it supports post, and likewise with GET

We should check the landing page and default to GET if it doesn't have a POST search link.

[philvarner]

maybe also have an override to force POST regardless of what's advertised

[jsignell]

Pairing this with #803 will minimize the impact on users.