Allow pgstac parameters for geojsonsearch to be globally set in Settings

[zstatmanweil]

Description

This closes #185. I have created a Settings object so the parameters to PgStac's geojsonsearch can be set globally via environment variables. I added an example to .env.example as well so hopefully it is clear how to set them. These can still also be set via query params.

Tests

I ran locally and made sure any change to these pgstac env variables would apply to any request I made. I then made sure I could pass in query params to a specific request and just that request would use that change. I confirmed those both by adding print statements in the code here, and working with my data that had issues when existwhenfull and skipcovered were both True. So easy for me to visually ensure what I expected to happen would happen.

[zstatmanweil]

@vincentsarago, question for you: when I open a PR in one of your projects, do I make updates to the CHANGES file at that time? Or do you update that when releasing a new version?

[zstatmanweil]

Also, I tried to follow instructions for running tests but I get this error for all of them: "psycopg.errors.FeatureNotSupported: extension "postgis" is not available"

[vincentsarago]

🙏 @zstatmanweil

@vincentsarago, question for you: when I open a PR in one of your projects, do I make updates to the CHANGES file at that time? Or do you update that when releasing a new version?

It depends, I always appreciate a sentence in a changelog within an Unreleased section

Also, I tried to follow instructions for running tests but I get this error for all of them: "psycopg.errors.FeatureNotSupported: extension "postgis" is not available"

Interesting, I'll have a look

[zstatmanweil]

I also added a line under Unreleased in the changes file

[vincentsarago]

🙏 thanks @zstatmanweil