tor-keymgr config updates

Previously, the keystore config consisted of a single field in StorageConfig, which encoded 2 bits of information: whether the keystore is enabled, and its root directory:

[storage]
# use this path, fail if compiled out
# keystore = "/path/to/arti/keystore"
#
# use default path, fail if compiled out
# keystore = true
#
# disable
# keystore = false

This commit adds ArtiNativeKeystoreConfig, which will replace the multi-purpose keystore field. The new config will look like this:

#[storage.keystore]
# Whether the keystore is enabled.
#
# If the `keymgr` feature is enabled and this option is:
#     * set to false, we will ignore the configured keystore path.
#     * set to "auto", the configured keystore, or the default keystore, if the
#     keystore path is not specified, will be used
#     * set to true, the configured keystore, or the default keystore, if the
#     keystore path is not specified, will be used
#
# If the `keymgr` feature is disabled and this option is:
#     * set to false, we will ignore the configured keystore path.
#     * set to "auto", we will ignore the configured keystore path.
#
# Setting this option to true when the `keymgr` feature is disabled is a
# configuration error.
#enabled = "auto"
# The root directory of the arti keystore
#path = "${ARTI_LOCAL_DATA}/keystore"

While ArtiNativeKeystoreConfig currently only has 2 fields, enabled and path, future versions of the keystore might require additional config options.