1600 allow providing default value in config

[ricardogsilva]

Overview

This PR implements allowing default values for config variables that can come from the environment. Most notably it becomes possible to combine several env variables in a single config value.

For the most common case of having the full variable correspond to some env value or else a default value, the syntax is myvar: ${MY_VAR:-some default}, as shown below:

server:
    host: ${MY_HOST:-localhost}
    port: ${MY_PORT:-5000}
    combined: ${MY_HOST:-localhost} - ${MY_PORT:-5000}

UPDATE: With the latest commit, this paragraph below is no longer relevant - no need to use the !env YAML tag

For cases where the env variable is being used as part of a config variable and it shows up in the middle of the variable it becomes mandatory to use the !env YAML tag:

metadata:
    identification:
        title:
            en: !env This is pygeoapi host ${MY_HOST} and port ${MY_PORT:-5000}, nice to meet you!

Related Issue / discussion

• Allow providing a default value in config when env variable does not exist #1600

Dependency policy (RFC2)

• I have ensured that this PR meets RFC2 requirements

Updates to public demo

• I have ensured that breaking changes to the pygeoapi master demo server have been addressed
  • https://github.com/geopython/demo.pygeoapi.io/blob/master/services/pygeoapi_master/local.config.yml

Contributions and licensing

(as per https://github.com/geopython/pygeoapi/blob/master/CONTRIBUTING.md#contributions-and-licensing)

• I'd like to contribute [feature X|bugfix Y|docs|something else] to pygeoapi. I confirm that my contributions to pygeoapi will be compatible with the pygeoapi license guidelines at the time of contribution
• I have already previously agreed to the pygeoapi Contributions and Licensing Guidelines

[ricardogsilva]

Failing CI seems to be due to ubuntugis-ppa being unavailable, it is not related to the changes in this PR, which do not touch any CI or dependency-related things

[kalxas]

Launchpad is down for a few hours for maintenance reasons

https://twitter.com/launchpadstatus/status/1770482383453577682

[tomkralidis]

Looks like Launchpad came back. @ricardogsilva can you check/fix the latest CI failures?

[ricardogsilva]

@tomkralidis I'm not able to ask for a re-run of the failed CI workflow - can you please do it?

[tomkralidis]

The CI was already re-run, and had some unit test errors. I've re-run them just now: https://github.com/geopython/pygeoapi/actions/runs/8365396387/job/22911691167?pr=1604#step:20:134

[ricardogsilva]

@tomkralidis fixed CI failures