Make behave more like built-in transports

[adsr]

I haven't found documentation on the apt transport protocol, so I'm just going by observed behavior of the built-in transports.

Make the following changes:

• Exit clean on EOF instead of running forever
• On malformed message, exit 100 without a protocol response
• Ignore unhandled message codes
• Like other transports, support a debug mode Debug::Acquire::gar

The http transport seems to consider an empty message an error, but given this case was already being handled specifically, I left it in.

[google-oss-prow[bot]]

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[adsr]

• Exit clean on EOF instead of running forever

We observed this behavior today. If the process gets orphaned somehow, it busy loops reading EOF on stdin forever. Example:

user@host:~$ sudo strace -fvyyYtttT -p 1327075 2>&1
strace: Process 1327075 attached with 20 threads
...
[pid 1327087<ar+https>] 1710535486.854801 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000036>
[pid 1327087<ar+https>] 1710535486.854859 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000010>
[pid 1327087<ar+https>] 1710535486.854893 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000012>
[pid 1327087<ar+https>] 1710535486.854930 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000011>
[pid 1327087<ar+https>] 1710535486.854971 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000010>
[pid 1327087<ar+https>] 1710535486.855008 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000012>
[pid 1327087<ar+https>] 1710535486.855048 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000012>
[pid 1327087<ar+https>] 1710535486.855087 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000011>
[pid 1327087<ar+https>] 1710535486.855124 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000010>
[pid 1327087<ar+https>] 1710535486.855156 read(0<pipe:[157797980]>, "", 4096) = 0 <0.000010>
...

[ericdand]

https://github.com/GoogleCloudPlatform/artifact-registry-apt-transport/pull/24 supersedes this one, and so this PR can now be closed; its changes have all been merged in. :)