Add keyPrefix support for TiKV store (#7756)

* Add keyPrefix support for TiKV store

Similar to the Redis keyPrefix feature (#7299), this adds keyPrefix support
for TiKV stores to enable sharing a single TiKV cluster as metadata store
for multitenant SeaweedFS clusters.

Changes:
- Add keyPrefix field to TikvStore struct
- Update Initialize function to read keyPrefix from config
- Add getKey method to prepend prefix to all keys
- Update generateKey, getNameFromKey, and genDirectoryKeyPrefix methods
  to be store receiver methods and handle key prefixing
- Update filer.toml scaffold with keyPrefix configuration option

Fixes #7752

* Fix potential slice corruption in getKey method

Use a new slice with proper capacity to avoid modifying the
underlying array of store.keyPrefix when appending.

* Add keyPrefix validation and defensive bounds check

- Add validation in Initialize to reject keyPrefix longer than 256 bytes
- Add bounds check in getNameFromKey to prevent panic on malformed keys

* Update weed/filer/tikv/tikv_store.go

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update weed/command/scaffold/filer.toml

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

weed/command/scaffold/filer.toml

@@ -399,6 +399,8 @@ enabled = false
 # If you have many pd address, use ',' split then:
 #   pdaddrs = "pdhost1:2379, pdhost2:2379, pdhost3:2379"
 pdaddrs = "localhost:2379"
+# prefix for filer TiKV keys, useful for sharing a TiKV cluster with multiple seaweedfs clusters
+keyPrefix = ""
 # Enable 1PC
 enable_1pc = false
 # batch delete count, default 10000 in code