bundle-uri: create "key=value" line parsing

bundle-uri: create "key=value" line parsing

When advertising a bundle list over Git's protocol v2, we will use
packet lines. Each line will be of the form "key=value" representing a
bundle list. Connect the API necessary for Git's transport to the
key-value pair parsing created in the previous change.

We are not currently implementing this protocol v2 functionality, but
instead preparing to expose this parsing to be unit-testable.

Co-authored-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>