Add support for new fixer API (fixes #14911)

[fabaff]

Description:

Fixer.io updated their API and now it's required to have an API key. Support for SSL/TLS is only available for paying customers.

Related issue (if applicable): fixes #14911

Pull request in home-assistant.github.io with documentation (if applicable): home-assistant/home-assistant.io#5654

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: fixer
    api_key: !secret fixer_api
    target: CHF

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:

• New dependencies have been added to the REQUIREMENTS variable (example).
• New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.

[fabaff]

Breaking change:
Fixer.io now requires you to have an API key to use their service. Please check the fixer sensor documentation for an update of your configuration as base is no longer supported but api_key required.

[MartinHjelmare]

Looks good!

[MartinHjelmare]

Maybe also mention removal of base key in options in the breaking change notes.

[fabaff]

Note updated.

[metbril]

The API docs still mention ‘base’ as optional.

https://fixer.io/documentation#latestrates

Why has this been removed from the component?

Also the scan interval is 1 day by default. The free API allows 1 call per hour. The scan interval can be increased if we make 1 call for all rates at once.

This would require changes to the configuration however,

sensor:
  - platform: fixer
    ## https://www.home-assistant.io/components/sensor.fixer/
    target: 
      - USD
      - CHF
    base: EUR
    name: Exchange Rate

Where there is only one 'base' for all rates and 'name' will become a prefix for any rate. Like 'exchange_rate_usd' and 'exchange_rate_chf'. Name could have a seensible default or be empty (no prefix).