Add support for multiple servers in config

[yanokwa]

I work with lots of different servers and it's a pain to constantly edit .pyodk_config.toml. I'd love to be able to switch profiles on the fly.

Given this .pyodk_config.toml, I'd like to be able to call Client(profile="demo") to use the demo url/user/pass and Client(profile="test") to use the test profile.

[demo]
base_url = "https://demo.getodk.cloud"
username = "alice"
password = "bob"

[default]
base_url = "https://www.example.com"
username = "my_user"
password = "my_password"
default_project_id = 123

[test]
base_url = "https://test.getodk.cloud"
username = "foo"
password = "bar"

Note that I have [default] for the default profile. Our docs for the single server config currently uses [central], so maybe if there is only one profile, that is the default regardless of what the header is. If there are two or more, then the default needs to be called [default]. Then we can update the docs to use [default].

[lindsay-stevens]

It's possible to do this kind of thing by specifying the config paths in the Client constructor, or via env vars:

Setting environment variables PYODK_CONFIG_FILE and PYODK_CACHE_FILE
Init arguments: Client(config_path="my_config.toml", cache_path="my_cache.toml").

So for 3 different servers the calls might look like:
Client(config_path="pyodk_default_config.toml", cache_path="pyodk_default_cache.toml")
Client(config_path="pyodk_demo_config.toml", cache_path="pyodk_demo_cache.toml")
Client(config_path="pyodk_test_config.toml", cache_path="pyodk_test_cache.toml")

Using separate files somewhat reduces the risk of accidentally copying credentials to somewhere they shouldn't be, and allows use of file system access controls with automation (e.g. script A user can see conf A files).