Support HTTP proxy

[yahavi]

• All tests passed. If this feature is not already covered by the tests, I added new tests.

Fix #59

Proxy is already supported before that PR by adding the following system properties to the Maven command:

-Dartifactory.proxy.host=acme.jfrog.io 
-Dartifactory.proxy.port=8888 
-Dartifactory.proxy.username=elmar 
-Dartifactory.proxy.password=Wabbit

This PR enables configuring HTTP proxy in one of the following new ways:

1. By the regular config:

<proxy>
    <host>acme.jfrog.io</host>
    <port>8888</port>
    <username>elmar</username>
    <password>Wabbit</password>
</proxy>

2. By settings.xml:

<proxies>
    <proxy>
        <id>jfrog-proxy</id>
        <host>acme.jfrog.io</host>
        <port>8888</port>
        <username>elmar</username>
        <password>Wabbit</password>
    </proxy>
</proxies>

[cb645j]

When will this be available? Im using the JFrogMaven@1 from azure devops extensions, i believe the JFrogMaven@1 uses this maven plugin, however it still does not read the proxy from my settings..