Add extra.branch-alias setting to composer.json

[phansys]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets
License	MIT
Doc PR

Add extra.branch-alias setting to composer.json in order to allow install
development versions while respecting semver.
This obviously can be updated taking into account the current branching/release
strategy, maybe the branch alias should be pointing to 0.5-dev instead of
0.4-dev.
This change allows to use constraints like "microsoft/windowsazure": "^0.4@dev", which will actually install thedev-master` version.

[azuresdkci]

Can one of the admins verify this patch?

[azurecla]

Hi @phansys, I'm your friendly neighborhood Azure Pull Request Bot (You can call me AZPRBOT). Thanks for your contribution!
You've already signed the contribution license agreement. Thanks!
We will now validate the agreement and then real humans will evaluate your PR.

TTYL, AZPRBOT;

[yaqiyang]

@phansys, Thanks very much for your contribution. However, I am not sure I understand your reasoning for adding this property. According to https://getcomposer.org/doc/04-schema.md#extra, this is to add extra data for the scripts property. But we don't have a scripts property here.
Also, will the data at the extra tag provide value for most users?

Thanks
Yaqi Yang

[phansys]

@yaqiyang, please take a look at https://getcomposer.org/doc/articles/aliases.md#branch-alias, I think it should explain the feature better than me.
Let me know if your need some clarification.
Cheers.

[phansys]

Thank you @yaqiyang.