Refactor configuration to use multiple sections

Currently the configuration file is a monolith in a single section [GWDataFindServer], as a legacy of this being part of a larger application.

We should refactor the configuration file parsing to enable multiple sections for

[auth]
authorization = scitokens

[cache]
path = /var/lib/diskcache/ascii_cache.dat
timeout = 10

[scitokens]
issuer = https://cilogon.org/igwn
audience = TEST
scope = gwdatafind.read

[x509]
gridmap = /etc/security/grid-mapfile
timeout = 60

[filter]
site_include_pattern =
site_exclude_pattern =
frametype_include_pattern =
frametype_exclude_pattern =
filter_preference = """<>"""

[osdf test]
...

This should hopefully make things more flexible, and easier to document.