support passing raw toml config for tiflash (pingcap#3355)

follow up pingcap#3327

origin way to set config:
```yaml
    tiflash:
      baseImage: pingcap/tiflash
      maxFailoverCount: 3
      replicas: 1
      storageClaims:
        - resources:
            requests:
              storage: 1Gi
--        # storageClassName: local-storage
      # requests:
      #   storage: "1Gi"
      config:
        config:
          logger:
            level: "warn"
            count: 9
        proxy:
          log-level: "warn"
          gc:
            batch-keys: 501
```
after this pr, support pass config like this:
```yaml
    tiflash:
      baseImage: pingcap/tiflash
      maxFailoverCount: 3
      replicas: 1
      storageClaims:
        - resources:
            requests:
              storage: 1Gi
--        # storageClassName: local-storage
      # requests:
      #   storage: "1Gi"
      config:
        config: |
          [logger]
            level = "warn"
            count = 9
        proxy: |
          log-level = "warn"
          [gc]
            batch-keys = 501
```

docs/api-references/docs.md

@@ -12757,12 +12757,35 @@ kubectl create secret generic <clusterName>-tidb-client-secret &ndash;namespace=
 </tr>
 </tbody>
 </table>
-<h3 id="tiflashconfig">TiFlashConfig</h3>
+<h3 id="tiflashcommonconfigwraper">TiFlashCommonConfigWraper</h3>
 <p>
 (<em>Appears on:</em>
-<a href="#tiflashspec">TiFlashSpec</a>)
+<a href="#tiflashconfigwraper">TiFlashConfigWraper</a>)
 </p>
 <p>
+</p>
+<table>
+<thead>
+<tr>
+<th>Field</th>
+<th>Description</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>
+<code>GenericConfig</code></br>
+<em>
+github.com/pingcap/tidb-operator/pkg/util/config.GenericConfig
+</em>
+</td>
+<td>
+</td>
+</tr>
+</tbody>
+</table>
+<h3 id="tiflashconfig">TiFlashConfig</h3>
+<p>
 <p>TiFlashConfig is the configuration of TiFlash.</p>
 </p>
 <table>
@@ -12803,6 +12826,74 @@ ProxyConfig
 </tr>
 </tbody>
 </table>
+<h3 id="tiflashconfigwraper">TiFlashConfigWraper</h3>
+<p>
+(<em>Appears on:</em>
+<a href="#tiflashspec">TiFlashSpec</a>)
+</p>
+<p>
+</p>
+<table>
+<thead>
+<tr>
+<th>Field</th>
+<th>Description</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>
+<code>config</code></br>
+<em>
+<a href="#tiflashcommonconfigwraper">
+TiFlashCommonConfigWraper
+</a>
+</em>
+</td>
+<td>
+</td>
+</tr>
+<tr>
+<td>
+<code>proxy</code></br>
+<em>
+<a href="#tiflashproxyconfigwraper">
+TiFlashProxyConfigWraper
+</a>
+</em>
+</td>
+<td>
+</td>
+</tr>
+</tbody>
+</table>
+<h3 id="tiflashproxyconfigwraper">TiFlashProxyConfigWraper</h3>
+<p>
+(<em>Appears on:</em>
+<a href="#tiflashconfigwraper">TiFlashConfigWraper</a>)
+</p>
+<p>
+</p>
+<table>
+<thead>
+<tr>
+<th>Field</th>
+<th>Description</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td>
+<code>GenericConfig</code></br>
+<em>
+github.com/pingcap/tidb-operator/pkg/util/config.GenericConfig
+</em>
+</td>
+<td>
+</td>
+</tr>
+</tbody>
+</table>
 <h3 id="tiflashspec">TiFlashSpec</h3>
 <p>
 (<em>Appears on:</em>
@@ -12928,8 +13019,8 @@ TiFlash supports multiple disks.</p>
 <td>
 <code>config</code></br>
 <em>
-<a href="#tiflashconfig">
-TiFlashConfig
+<a href="#tiflashconfigwraper">
+TiFlashConfigWraper
 </a>
 </em>
 </td>

go.mod

@@ -93,6 +93,7 @@ require (
 	k8s.io/utils v0.0.0-20190801114015-581e00157fb1
 	sigs.k8s.io/apiserver-builder-alpha/cmd v0.0.0-20191113095113-4493943d2568
 	sigs.k8s.io/controller-runtime v0.4.0
+	sigs.k8s.io/yaml v1.1.0
 )
 
 replace github.com/renstrom/dedent => github.com/lithammer/dedent v1.1.0