Add support for short-lived tokens (#224)

The setup-gradle action tries to get a short-lived access token given the supplied Develocity access key.
This key can be passed either with the `DEVELOCITY_ACCESS_KEY` env var or via the  `develocity-access-key` input parameter.
If a token can be retrieved, then the `DEVELOCITY_ACCESS_KEY` env var will be set to the token. 
Otherwise the `DEVELOCITY_ACCESS_KEY` will be set to a blank string, to avoid a leak.

---------

Co-authored-by: daz <daz@gradle.com>

sources/src/develocity/build-scan.ts

@@ -0,0 +1,31 @@
+import * as core from '@actions/core'
+import {BuildScanConfig} from '../configuration'
+import {setupToken} from './short-lived-token'
+
+export async function setup(config: BuildScanConfig): Promise<void> {
+    maybeExportVariable('DEVELOCITY_INJECTION_INIT_SCRIPT_NAME', 'gradle-actions.inject-develocity.init.gradle')
+    maybeExportVariable('DEVELOCITY_AUTO_INJECTION_CUSTOM_VALUE', 'gradle-actions')
+    if (config.getBuildScanPublishEnabled()) {
+        maybeExportVariable('DEVELOCITY_INJECTION_ENABLED', 'true')
+        maybeExportVariable('DEVELOCITY_PLUGIN_VERSION', '3.17.3')
+        maybeExportVariable('DEVELOCITY_CCUD_PLUGIN_VERSION', '2.0')
+        maybeExportVariable('DEVELOCITY_TERMS_OF_USE_URL', config.getBuildScanTermsOfUseUrl())
+        maybeExportVariable('DEVELOCITY_TERMS_OF_USE_AGREE', config.getBuildScanTermsOfUseAgree())
+    }
+    setupToken(
+        config.getDevelocityAccessKey(),
+        config.getDevelocityTokenExpiry(),
+        getEnv('DEVELOCITY_ENFORCE_URL'),
+        getEnv('DEVELOCITY_URL')
+    )
+}
+
+function getEnv(variableName: string): string | undefined {
+    return process.env[variableName]
+}
+
+function maybeExportVariable(variableName: string, value: unknown): void {
+    if (!process.env[variableName]) {
+        core.exportVariable(variableName, value)
+    }
+}