Retry external artifact downloads on transient network failures

[Copilot]

External artifact downloads in maven-external-dependency-plugin failed the build on a single transient java.net.ConnectException: Connection timed out, since wagon.get(...) was invoked exactly once with a 5s default timeout.

Changes

• Retry loop around Wagon download (ResolveExternalDependencyMojo): up to N attempts, WARN-logged with attempt counter and cause, wagon.disconnect() in finally, partial temp file deleted between attempts, Thread.sleep(retryDelay) between attempts (interrupt-safe). On exhaustion throws MojoExecutionException("Failed to download artifact <url> after N attempt(s)", lastFailure).
• Fail-fast on auth errors: AuthorizationException is not retried.
• New Mojo parameters with defaults: downloadRetryAttempts=5, downloadTimeout=10000, downloadRetryDelay=2000.
• Per-artifact overrides on <artifactItem>: timeout, retryAttempts, retryDelay — take precedence over Mojo-level defaults when explicitly set (preserves backward compatibility). Added ArtifactItem.getTimeoutRaw() so the resolver can distinguish "unset" from the existing defaulted getter.
• Default timeout in ArtifactItem.getTimeout() raised from 5s → 10s.
• README updated with the new parameters table and override semantics.

Example

<plugin>
  <groupId>com.savage7.maven.plugins</groupId>
  <artifactId>maven-external-dependency-plugin</artifactId>
  <configuration>
    <downloadRetryAttempts>5</downloadRetryAttempts>
    <downloadTimeout>10000</downloadTimeout>
    <downloadRetryDelay>2000</downloadRetryDelay>
    <artifactItems>
      <artifactItem>
        <!-- per-artifact override -->
        <retryAttempts>10</retryAttempts>
        <timeout>30000</timeout>
        ...
      </artifactItem>
    </artifactItems>
  </configuration>
</plugin>