Added minder provider update CLI command.

Update command allows changing configuration for provider on a
per-field basis. It operates on a read-modify-write fashion,
retrieving current configuration from the backend for modification.

Modification is done by reflection by walking over the struct's JSON
tags. Implementation is recursive, but the maximum depth is determined
by the deepest field in the configuration struct.

Argument parsing is trivial and assumes that arguments are either of
the form `field1.field2.field3` (for `--unset-attribute`) or
field1.field2.field3=value` (for `--set-attribute`). The right parser
for `value` is determined once the correct struct field is found.

It is not currently possible to modify `"github-app"` or `"github"`
fields.

Fixes #3509