Support for megav2 and megav3 with MEGACMD

[flowgunso]

New mega.nz schemes megav2:// and megav3:// can be supported using the megacmd utility. I've released a working image at flowgunso/volumerize-mega.

IMHO the two most viables options to provide the megacmd utility:

• Rely on danielquinn/megacmd-alpine to provide the compiled binairies and libraries, then install the dependencies. Which is what I've done. But it adds a dependency to the latter.
• Compile the megacmd utility. But it will add significant build time to the CI.

What do you think ?

In the meantime, I've added a Mega tutorial.

[blacklabelops]

I would prefer the compile version.

[flowgunso]

MegaCMD is compiled in a separate stage to reduce the image footprint. Based on @danielquinn's MEGAcmd for Alpine Linux.

[flowgunso]

Build error message is curl: (22) The requested URL returned error: 503. I've tried the three relevant curls and they're fine now.