feat(templates): adds arbitrary shell execution and pda-getting

testdata/template-enum-err.ct

@@ -2,4 +2,4 @@
 $ fecho tpl {{ enum .LEVEL "info" "warn" }}
 $ pda set level@tple < tpl
 $ pda get level@tple LEVEL=debug --> FAIL
-FAIL cannot get 'level@tple': template: cmd:1:3: executing "cmd" at <enum .LEVEL "info" "warn">: error calling enum: invalid value 'debug', allowed: [info warn]
+FAIL cannot get 'level@tple': invalid value 'debug', allowed: [info warn]

testdata/template-pda-ref-err.ct

@@ -0,0 +1,5 @@
+# pda errors on missing key
+$ fecho tpl1 {{ pda "missing" }}
+$ pda set ref@tplre < tpl1
+$ pda get ref@tplre --> FAIL
+FAIL cannot get 'ref@tplre': pda: cannot get 'missing': no such key

testdata/template-pda-ref.ct

@@ -0,0 +1,13 @@
+# pda function cross-references another key
+$ pda set base https://example.com
+$ fecho tpl1 {{ pda "base" }}/api
+$ pda set endpoint@tplr < tpl1
+$ pda get endpoint@tplr
+https://example.com/api
+# pda with substitution vars passed through
+$ fecho tpl2 Hello, {{ default "World" .NAME }}
+$ pda set greeting@tplr < tpl2
+$ fecho tpl3 {{ pda "greeting@tplr" }}!
+$ pda set shout@tplr < tpl3
+$ pda get shout@tplr NAME=Alice
+Hello, Alice!

testdata/template-require-err.ct

@@ -2,4 +2,4 @@
 $ fecho tpl {{ require .FILE }}
 $ pda set tmpl@tplr < tpl
 $ pda get tmpl@tplr --> FAIL
-FAIL cannot get 'tmpl@tplr': template: cmd:1:3: executing "cmd" at <require .FILE>: error calling require: required value is missing or empty
+FAIL cannot get 'tmpl@tplr': required value is missing or empty

testdata/template-shell.ct

@@ -0,0 +1,5 @@
+# Shell function executes a command and returns stdout
+$ fecho tpl1 {{ shell "echo hello" }}
+$ pda set shelltest@tpls < tpl1
+$ pda get shelltest@tpls
+hello